Electron discharge devices of the growing wave type



March 5, 1957 N. E. LINDENBLAD 2,784,339

ELECTRON DISCHARGE DEVICES OF THE GROWING WAVE TYPE Original Filed June 25. 1947 INVENTOR N35 Elvin 1811111 afl ORNEY United States Eatent G ELECTRON DISCHARGE DEVICES OF THE GRQWING WAVE TYPE Nils E. Lindenblad, Princeton, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Original application June 25, 1947, Serial No. 756,851, now Patent No. 2,578,434, dated December 11, 1951. and this application May 31, 1951, Serial No.

12 Claims. (Cl. 315-35) This invention relates to improvements in electron discharge devices of the growing wave type, especially adapted for use at ultra and very high frequencies.

This application is a division of copending application Serial No. 756,851, filed June 25, 1947, now U. S. Patent No. 2,578,434, issued December 11, 1951, assigned to the same assignee as the present application. The present invention is illustrated in Fig. 3 of said copending application.

The principal object of the present invention is to provide a wide band helical line type of growing wave amplifier tube for use at high frequencies in which the occurrence of parasitics at very high frequencies outside the range of frequencies to be amplified is prevented or minimized.

In accordance with the invention, a plurality of reactive impedance elements are coupled between the helical conductor and its surrounding shield along at least a substantial portion of the length of the conductor. Each of the impedance elements may, for example, take the form of a capacity plate capacitively coupled to the helical conductor and connected to the shield through an absorber resistor.

A detailed description of the invention follows, in conjunction with the drawing, in which the single figure is a longitudinal sectional view of a growing wave tube embodying the invention.

' Referring to the drawing in more detail, there is shown a Wide band growing wave amplifier tube of the broad general type described in my U. S. Patent No. 2,300,052, granted October 22, 1942, and in my copending application Serial No. 724,330, filed January 25, 1947, in which a helical conductor surrounds a stream of electrons and is in energy coupling relation thereto. The improved amplifier tube comprises a tubular non-magnetic metallic envelope or shell which surrounds a helical conductor or coil 11. A suitable cathode is provided at one end of the tube for furnishing a concentrated heavy stream of electrons which pass through the interior of the helical conductor 11 and which is finally collected by a collector electrode 12 at the other end of the tube. The cathode 13 is shown, by way of example, as comprising a cylinder 14 which has only a portion 15 thereof sprayed with electron emitting material. A repeller electrode 16 at the cathode end of the tube serves to repel or concentrate the electrons emitted by the cathode 13 toward the helical conductor 11. A magnetic field coil 17 surrounds the tube and is energized by a direct-current source 18 in series with a variable resistor 19. The field coil is so arranged that the lines of flux extend parallel to the tube in an axial direction in order to focus the beam of electrons along the center of the tube. It should be noted that the repeller electrode 16 is maintained at a negative potential relative to the cathode 13, while the collector electrode 12 is maintained at a slight positive potential relative to the cathode. In the construction of the tube it may be desirable for the collector electrode to be at a slight negative potential relative to the cathode or even at the same potential as the cathode. The metallic envelope 10 is at ground potential which is equivalent to a positive potential with respect to the cathode, A coaxial transmission line 20 supplies radio frequency input current to one terminal of the helical conductor 11, while the amplified current is abstracted from the other terminal of the helical conductor 11 by an output coaxial line 21.

In order to assure a vacuum tight shell or envelope 10, glass beads 22 are provided in the input and output coaxial lines at a location near shell 10. Obviously, the glass beads can be positioned at any suitable location.

The helical conductor 11 is a plurality of wavelengths long peripherally along the helix at the center frequency of operation. The input energy supplied to the helix 11 by input line 20 causes the electrons passing through the interior of the helix 11 to be bunched. The helix 11 has such dimensions as to couple properly with the electron stream passing along the axis.

The characteristic impedance of this helical conductor is of the order of several hundred ohms, whereas the characteristic impedances of the coaxial lines 20 and 21 are each of the order of 50 ohms, as an example. In order to match the impedances of the coaxial lines 20 and 21 to the characteristic impedance of the helical conductor 11, to prevent the production of standing waves due to reflections at the junctions between the lines 20 and 21 and the conductor 11, the metallic envelope 10, which functions as a shield around the conductor 11, is tapered in diameter toward its ends so as to reduce the characteristic impedance of the helical conductor 11 gradually toward its terminals until it is equal to that of the coaxial lines to which it is connected. In order to taper the impedance of the helical conductor 11 down to approximately 50 ohms, which is assumed to be the impedance of the coaxial lines, the distance between the last turn of the coil 11 and the surrounding tapering shell 10 should be of the order of the radius of the wire constituting the coil 11. Because this might be difficult to achieve in practical conditions due to the very close spacing required, it will sometimes be more convenient not to taper the characteristic impedance of the helical conductor 11 down as far as 50 ohms, but rather to a value of the order of to ohms, in which case the spacing between the last turn of the coil 11 and the tapering envelope 10 can be larger. In this last case it is advisable and preferred that a transmission line link having tapering impedance be inserted externally of the tube between the last turn of the coil 11 and the 50 ohm coaxial line.

In constructing the growing wave vacuum tube, it is advantageous to provide a coil 11 which has extremely low loss at the frequencies to be amplified, so that as much as possible of the kinetic energy of the bunched electrons can be transferred to the working load. At other frequencies, it is of course immaterial whether a loss occurs internally or externally of the tube. The arrangement, however, should be such that sutlicient loss is introduced to prevent parasitic oscillations.

The drawing shows an arrangement, employing the principles of the present invention, wherein parasitics can be prevented at very high frequencies outside the range to be amplified at which the conducting metallic shell 10 may provide undesired cavity resonance conditions. This is done by employing reactive impedance elements, for example, in the form of absorber resistors 50 connected at one end to the grounded shell 10 and at the other end to capacity plates 51. These plates 51 are spaced from and uniformly distributed along at least a substantial portion of the length of the helical conductor 11 and have such dimensions and are so spaced from the conductor 11 that the reactance between the conductor 11 and the plates is very high in the operating frequency range but becomes low at frequencies higher than the highest operating frequency. At frequencies higher than the operating frequency range, the low reactance between the helical conductor 11 and the capacity plates 51 will permit sutficient current to pass into the resistors 50 to introduce the desired damping effect to prevent the production of standing waves on the helical conductor 11.

In the operation of the tube as an amplifier, the beam electrons give up energy to the coil 11 as they travel therealong, and hence, their average speed is reduced. Although the helical conductor 11 has been shown as having the same pitch throughout its length, it should be noted that it may be tapered toward the end near the collector electrode in order to slow down the waves on the conductor 11, to maintain substantial synchronism between the Waves and the retarded beam electrons. The

potential on the collector electrode 12 will be held at suitable values depending on the speed of the electrons approaching it, and may be maintained at a low negative or a small positive potential relative to the cathode.

In some cases, it may be desired to maintain the helical conductor 11 at a particular potential relative to ground. In such a case the helix can be isolated from the input and output coaxial lines from a direct current standpoint, by means of blocking condensers, and the helix supplied with a suitable potential relative to the cathode.

The growing wave amplifier tube of the invention is particularly useful in a radio relaying system in amplifying an extremely wide band of radio frequency waves and wherein the mean frequency is of the order of many hundreds or thousands of megacycles.

What is claimed is:

1. An electron discharge device adapted to operate over a given frequency range, comprising a helical conductor, means for projecting a stream of charged particles along and in energy-transfer relation to said conductor, and reactive impedance means coupled to said ical conductor in spaced relation thereto, and reactive I impedance means coupled between said two conductors at a plurality of points therealong for by-passing high frequency current from said conductors substantially only at frequencies higher than the highest operating frequency of said range.

3. An electron discharge device comprising a helical conductor, means for projecting a stream of charged particles along and in energy-transfer relation to said conductor, and a plurality of reactive impedance elements particles along and in energy-transfer relation to said conductor, and a plurality of reactive impedance elements coupled at one end to said conductor at points uniformly distributed along at least a substantial portion of the length thereof, each of said impedance elements comprising a capacity element capacitively coupled to said conductor and connected to one terminal of a resistor element.

7. An electron discharge device according to claim 6, further including means connecting the other terminals of said resistor elements together.

8. An electron discharge device comprising a helical conductor, means for projecting a stream of charged particles through the interior of said conductor, a tubular electrostatic shield surrounding said helical conductor over at least the major portion of the length thereof and spaced therefrom, and a plurality of reactive impedance elements individually coupled between said tubular shield and said conductor, said impedance elements 1 being uniformly distributed along at least a substantial portion of the length of said helical conductor.

9. An electron discharge device according to claim 8, adapted to operate over a predetermined frequency range, wherein each of said impedance elements has a reactance value which is relatively high in said operating range and relatively low at frequencies higher than the highest operating frequency of said range.

10. An electron discharge device according to claim 8, wherein each of said impedance elements comprises a capacity element capacitively coupled to said helical conductor and connected to said tubular shield.

11. An electron discharge device according to claim 10, wherein the connection between each capacity element and said tubular shield includes an absorber resistor.

12. An electron discharge device adapted to operate over a predetermined frequency range, comprising a helical conductor through the interior of which a stream of charge particles is adapted to pass, a cylindrical metallic tube surrounding said helical conductor over substantially the entire length thereof and spaced therefrom, a plurality of capacity elements connected to said metallic tube along at least a portion of the length thereof and spaced from said helical conductor, and an absorber element connected between each capacity element and the metallic tube, the reactance value of said elements being coupled at one end to said conductor at points uniformly distributed along at least a substantial portion of the length thereof.

4. An electron discharge device according to claim 3,

relatively high in said operating range of said discharge device and relatively low at frequencies higher than the highest operating frequency of said range.

References Cited in the file of this patent UNITED STATES PATENTS 2,064,469 Haefi Dec. 15, 1936 2,575,383 Field Nov. 20, 1951 2,602,148 Pierce July 1, 1952 

